Hereinafter, conventional methods for manufacturing a monocrystalline silicon thin film will be described.
(a) Oxygen Ion Implantation Method
Oxygen ions are implanted into a monocrystalline silicon substrate, followed by heat treatment, so that a layered structure is formed which is composed of monocrystalline silicon, silicon dioxide, and the monocrystalline silicon substrate.
However, when oxygen ions are implanted into the monocrystalline silicon substrate, problems may arise in that many defects are generated in upper-side monocrystalline silicon and the cost of ion implantation is high (see Patent Document 1 below).
(b) Hydrogen Ion Implantation Method
After hydrogen ions (H+ and H−) are implanted into a monocrystalline silicon substrate, this substrate is adhered to a support substrate, followed by heat treatment. Subsequently, a layer implanted with the hydrogen ions is destroyed and is then peeled away, so that a monocrystalline silicon thin film having a thickness on the order of submicrons can be formed on the support substrate.
Since implanted hydrogen can only reach a depth on the order of submicrons, for example, in solar cell applications, the thickness of the monocrystalline silicon thin film must be increased to approximately 10 μm by a chemical vapor deposition or a physical vapor deposition method at a temperature of 1,000° C. or more. However, it is difficult to obtain an inexpensive substrate which can satisfy requirements for heat resistance and coefficient of thermal expansion. In addition, a method for increasing the thickness of a monocrystalline silicon thin film before a hydrogen ion implanted layer is peeled away from a substrate cannot be realized since the hydrogen ion implanted layer is destroyed under the film-thickening conditions (see Patent Document 2 below).
(c) Porous Silicon Method
When a surface of a monocrystalline silicon substrate is anodized, fine pores can be formed at a high density. After oxidation treatment is performed on surfaces of the fine pores thus formed, followed by removing parts of oxide layers which are close to the external surface with hydrofluoric acid, annealing is performed in a hydrogen atmosphere. As a result, the top-most surface again forms a continuous monocrystalline film, so that a structure containing a great number of voids is formed thereunder. After this substrate thus treated is adhered to a support substrate, when the layer containing voids is chemically dissolved using a liquid phase method or is mechanically destroyed by water jet or the like, a monocrystalline silicon thin film can be separated (see Patent Document 3 below).
However, the thickness of the upper-side silicon film is only approximately 1 μm which is a thickness to be contributed by the surface tension, and when this silicon is used in solar cells, the thickness must be increased by a CVD method. Furthermore, when peeling is performed by mechanical destruction, the monocrystalline silicon substrate is also damaged, and hence a problem may arise in some cases in that the repeated use of the monocrystalline silicon substrate is limited. In addition, a large number of steps are required, and the process is also disadvantageously complicated.
(d) Melting Recrystallization Method/Melting Crystallization Method
When a silicon dioxide film, a polycrystalline or an amorphous silicon thin film, and a protective layer made of silicon dioxide are laminated in that order on a silicon substrate, and scanning of a line-shaped melting zone by lamp heating or the like is performed, a polycrystalline silicon thin film can be formed in which crystal grains are well grown in the in-plane direction. Subsequently, after the protective layer is chemically dissolved, and the thickness of the polycrystalline silicon thin film is increased by a CVD method, etching of the silicon dioxide film is performed with hydrofluoric acid, so that the polycrystalline silicon thin film can be separated (see Patent Document 4 below).
However, since the thin layer thus obtained is merely a polycrystalline silicon thin film, besides inferior energy conversion efficiency, the silicon substrate is also disadvantageously degraded while the molten zone is scanned, and in addition, the process is complicated due to a great number of manufacturing steps.
(e) Epitaxial Lift-Off (ELO) Method Using Sacrificial Layer Having Different Elemental Composition
An epitaxial lift-off (ELO) method is a method for obtaining a monocrystalline thin film, that is a target material, comprising the steps of preparing a monocrystalline substrate used as a template, forming a sacrificial layer thereon by epitaxial growth, forming a target film on the sacrificial layer by epitaxial growth, and removing the sacrificial layer.
Incidentally, when being used for solar cells, a monocrystalline silicon thin film exhibits superior energy conversion efficiency, safety, stability and the like; however, the cost is disadvantageously high. As ultra pure silicon used for solar cells, substandard silicon which is a part of silicon produced in semiconductor industries has been procured at an inexpensive price; however, the ratio of the cost of silicon substrates is still high, and in addition, in recent years, the surplus of silicon in semiconductor industries is getting short to meet the rapidly growing silicon consumption in the solar cell industry. Accordingly, when the substrates made from monocrystalline silicon can be replaced with thin films made therefrom, problems of the cost and supply of raw materials can be solved.
Accordingly, the inventor of the present invention proposed a method for manufacturing a monocrystalline silicon thin film by an ELO method (see Patent Document 5 below). In this method, it was proposed that when a metal silicide, a doped silicon layer or the like, that is “a material having a different elemental composition”, is used as a sacrificial layer, the ELO method may also be applied to silicon.
In particular, on a monocrystalline silicon substrate, a layer having a composition different from pure silicon, that is, metal silicide or highly doped silicon is epitaxially-grown as the sacrificial layer (intermediate layer), and silicon is then further epitaxially-grown thereon to form a monocrystalline silicon thin film, followed by removal of the sacrificial layer by chemical etching, so that the monocrystalline silicon substrate and the monocrystalline silicon thin film are separated from each other. Accordingly, the method was proposed in which while the monocrystalline silicon substrate is being reused, the monocrystalline silicon thin films are manufactured.
However, the method using the sacrificial layer as described above also has problems. That is, when a material such as metal silicide is used, impurities are incorporated into the monocrystalline silicon thin film, and hence a problem of the decreased energy conversion efficiency of solar cells occurs. On the other hand, when a doped silicon layer is used, in the process of forming the monocrystalline silicon thin film by epitaxial growth, a dopant diffuses in the monocrystalline silicon thin film and the substrate directions, and as a result, a problem has occurred in that a highly doped layer cannot be maintained.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-077352
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-040785
Patent Document 3: Japanese Unexamined Patent Application Publication No. 05-275663
Patent Document 4: Japanese Unexamined Patent Application Publication No. 07-226528
Patent Document 5: WO0240751